Illumination systems may be found in many different applications, including image projection display systems, backlights for liquid crystal displays and the like. Projection systems usually use a source of light, illumination optics to pass the light to one or more image-forming devices, projection optics to project the image(s) from the image-forming device(s) and a projection screen on which the image is displayed. The image-forming device(s) are controlled by an electronically conditioned and processed video signal.
White light sources, such as high-pressure mercury lamps, have been, and still are, the predominant light sources used in projection display systems. In a three-panel image-projection system, the white light beam is split into three primary color channels, red, green and blue, and is directed to respective image-forming device panels that produce the image for each color. The resulting primary-colored image beams are combined into a full color image beam that is projected for display. Some other projection systems use a single imager panel, and so rotating color wheels, or some other type of time-sequential color filter, is used to filter the white light so that light at one primary color is incident on the image-display device at any one time. The light incident at the panel changes color sequentially to form colored images synchronously with the incident light. The viewer's eye integrates the sequentially colored images to perceive a full color image.
More recently, light emitting diodes (LEDs) have been considered as an alternative to white light sources. For a three panel system, an array of red LEDs is used to illuminate the red channel, an array of green LEDs is used for the green channel and an array of blue LEDs is used for the blue channel. Some advantages of using LED light sources include longer lifetime, higher efficiency and superior thermal characteristics.
LED-based illumination systems display a color gamut that is different from that recommended by the Society of Motion Picture and Television Engineers in Recommended Practices 145-1999, “C Color Monitor Colorimetry” (referred to hereafter as SMPTS 145-1999). Also, in order to achieve an acceptable white color, LED-based illumination sources are often run in a mode that fails to realize the full intensity available from all LEDs, so the overall brightness of the image is reduced.